I've got "the one" ski for those typical mixed resort days. They are probably the right choice 6 out of 10 days, and definitely what I would bring on a trip that involved air travel.
Two out of 10 days are best enjoyed on the carving skis, and 2 out of ten are best on the fatties.
Do I need those two additional pair? Of course not.
Do they add to the enjoyment of one of my greatest passions? Absolutely!
A buddy said I should buy my next ski for the terrain I want to ski. Here in the PNW, skis seem to range from 70 mm on the groomers to 130 mm in the POW. Mathematically, the one, "most-of-the mountain, most-of-the-time" ski would fall between 90 and 110 mm under foot. I know this may seem like an oversimplification, but, hey, I'm a simple guy. Individual differences in ability and style will seek out the best suited design. That is a long winded way of saying I concur with your 60% observation. One ski, though, that handles all conditions with the same effort and feel? I guess if the manufacturers claim to deliver, people will flock to buy.